Scanners are Input Devices that capture physical documents, photographs, illustrations, objects or other visual information and convert them into digital data. A conventional document scanner illuminates a physical page and uses an image sensor to measure reflected light as the scanning mechanism moves across the document. The resulting image is sampled and digitised, producing a bitmap that can be stored, displayed, processed or subjected to optical character recognition. Different scanner designs are optimised for flat documents, photographs, books, film, three-dimensional objects or high-volume document processing.
Flatbed scanners use a glass platen on which the source document is placed, with an optical assembly moving beneath or above the document during scanning. Sheet-fed scanners instead move individual sheets through a stationary or partially moving imaging system and are particularly useful for high-volume document workflows. Drum scanners use a different optical architecture and have historically been associated with high-end professional image reproduction. Handheld and specialised scanners provide other form factors for situations where portability or unusual source materials matter. The physical mechanism strongly influences speed, resolution, document handling and image quality.
Image sensors in scanners commonly use charge-coupled device or complementary metal-oxide-semiconductor technologies, depending on the design. The sensor converts incoming light into electrical signals, which are then amplified, sampled and converted into digital values. Colour scanners typically capture multiple colour channels, either through separate sensor paths, sequential illumination or integrated colour-sensitive arrangements. Optical resolution describes the physical sampling capability of the imaging system, while interpolated resolution is produced computationally and does not represent additional captured detail. This distinction is important when comparing scanner specifications.
Scanner resolution is commonly expressed in dots per inch, or DPI, although technically the sensor samples image information rather than printing dots in the same way as a printer. Higher optical resolution can capture finer detail but increases file size, scanning time and processing requirements. Colour depth determines how many distinct intensity levels can be represented for each channel; greater bit depth can preserve more tonal information, particularly during image editing. Dynamic range is another important characteristic because it determines how effectively a scanner can distinguish subtle differences between dark and light areas.
Scanners are widely used for document digitisation, archival projects, office workflows, photography, publishing, healthcare, manufacturing and research. Automatic document feeders allow large collections of pages to be scanned with limited manual intervention. Duplex scanners can capture both sides of a sheet during a single pass. Barcode and form-recognition systems can extract structured information from scanned documents, while optical character recognition can convert printed text into machine-readable characters. This allows scanned documents to become searchable and editable rather than remaining simple images.
Optical character recognition is a software process that commonly follows scanning when the goal is to extract text. The scanner captures pixels, but OCR analyses visual patterns and attempts to identify characters, words and document structure. Accuracy depends on image quality, font characteristics, language, layout, noise and the sophistication of the recognition system. Scanning at an unnecessarily low resolution can make OCR less accurate, while excessively high resolution increases processing and storage requirements without necessarily improving recognition. Image preprocessing such as deskewing, noise reduction, thresholding and contrast adjustment can significantly affect results.
Scanning is also used beyond conventional flat documents. Film scanners are designed around the optical characteristics and small physical dimensions of photographic film, while three-dimensional scanners capture the geometry or surface characteristics of objects using optical, laser or structured-light methods. These specialised devices are related conceptually to document scanners but should not be treated as identical technologies. Their sensors, illumination systems, calibration procedures and output representations are designed around fundamentally different measurement problems.
Scanner limitations include sensitivity to dust, scratches, paper quality, optical alignment, mechanical wear and imperfect source documents. High-end scanners can produce extremely detailed images but may be expensive and slow, while inexpensive office scanners prioritise convenience and throughput. Security and privacy are relevant because scanning can digitise sensitive personal, financial, medical or business information. A scanner therefore serves as the bridge between physical information and digital processing, and its usefulness depends on the complete chain from illumination and optical capture through digitisation, image processing, storage, OCR and document-management systems.
